# Graphing Question

## Recommended Posts

Hi I am using A U12 and DAQ Express to monitor my solar and wind input to my battery bank and the current being used by the inverter. This is being measured using Ratiomatic Hall Effect sensors.

I plot the thee variables SOLAR, WIND, INVERTER but would like to plot the calculated NET loss/Gain. I have created a V channel but am unable to see it on the graph. NET is not on the drop down list.

Can somebody explain how to derive a calculated value and graph it and maybe what i am doing wrong.

Andy

##### Share on other sites

There are actually two ways to do this. If you use a V channel, you have to remember to identify it with V. in front, so if you called the V channel NET, you'd want to put:

V.NET

in the Y Expression for the graph. Also, make sure the expression in the V channel is an array calculation. This generally means that you don't use [0] anywhere. So, if NET is SOLAR + WIND - INVERTER, you would put:

Solar + Wind - Inverter

and NOT:

Solar[0] + Wind[0] - Inverter[0]

as this second expression would result in a scalar. You can tell you have an array by going to the table tab of the V channel. If it shows a bunch of data points with different times, then you have it right and can graph it.

The other way to do it is to simply put the expression into the Y Expression of the graph. DAQFactory is not like Excel or other packages where you have to put calculations into some sort of variable first, and then graph it. In DAQFactory, you can graph calculated data right from the graph. So, you can simply put the expression you put in your V channel into the graph.

The reason for using a V channel would be if you needed to use this same expression in multiple places. Then, you could put the expression in the V channel and reference the V channel from multiple places. Then if you want to change the calculation, you can do it in one place, in the V channel, and it will update everywhere.